[warp]

https://opensource.org/licenses/category is the list of all Open Source licenses vetted by the OSI (open source initiative).

Other licenses may be open source, but you'd probably have to get lawyers involved to make sure. So it's better to just pick a license which the OSI considers Open Source.

If you don't want to rely on just the OSI, you can also check what the Free Software Foundation, Debian and Red Hat think of the license you've picked.

[account42]

Moreover, picking an existing popular open source license is important because a and b being opensource licenses does not mean that they are compatible, i.e. they could still have mutually-incompatible requirements that prevent anyone from releasing a combination of a-licensed and b-licensed code.